Do cockroaches eat ants? (Robert, San Jose, CA, U.S.A.)

Let me expand on this a bit. Over the last several years, I have noticed a strip of sidewalk near my home where both cockroaches and Argentine ants seem to congregate. Often times at night when the temperatures are mild, I'll go for a stroll on this particular strip of sidewalk and I'll notice a great number of cockroaches (all sizes large and small) traveling and loitering among the ant trails. What do you suspect is going on here?

Thanks,

Robert in San Jose

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Hi Robert,

Thanks for your question!

In all likelihood, the association you observed between the cockroaches & ants is merely coincidental rather than predatory. American cockroaches and Argentine ants alike are omnivorous scavengers that feed on a wide variety of organic matter (almost invariably dead if animal-based) and take eager advantage of the breadth of available foodstuffs in and around our homes. As with most domestic insect pests, they are more or less trophic equals, exploiting the same basic resources with similarly indiscriminate fervor. The concentration of these two opportunistic feeders in the same place says more about their overlapping diets than anything else, leading one to suspect that if you followed this particular nighttime procession, you might find some discarded fruit or an unattended waste bin or any number of other mutually appetizing treats.

While there are no known instances of cockroaches actively preying on ants, the scientific literature offers several examples of these two organisms engaging in other unexpected ways. Facultative myrmecophily (literally "ant-love", describing any meaningful association between ants and other organisms) has been documented in several species of cockroaches ranging from the miniscule Myrmecoblatta wheeleri, which has been found in the nests of both Solenopsis and Camponotus ants (Hebard, 1917; Fisk et al., 1976; Deyrup & Fisk, 1984), to the parthenogenetic Surinam cockroach, Pycnoscelus surinamensis, which exhibits a strong commensal association with the rover ant Brachymyrmex cordemoyi (Moretti et al., 2011). An earlier study describes additional, if somewhat less permanent, associations between P. surinamensis and seven other ant species, primarily the widespread tramp ant species Pheidole megacephala and Paratrechina longicornis (Deleport et al., 2002).

Perhaps the best-known cockroach myrmecophiles are those in the genus Attaphila, which maintain close host-specific associations with fungus-growing ants in the genera Atta and Acromyrmex. Most extensively studied of these is Attaphila fungicola (pictured), a minute cockroach that inhabits the fungus gardens of Atta texana and obtains nourishment by licking the exteriors of passing soldiers (Wheeler, 1910). These cockroaches will also instinctively mount the males and queen ants before a mating flight, thereby ensuring a wider dispersal than would otherwise be possible on their own.

Photo courtesy of LSU AgCenter

In many cases, myrmecophilous cockroaches display a marked ability to detect and orient to ant trail pheromones, especially in the more established Attaphila-Attine relationships. While it's certainly possible that the cockroaches you observed were somehow attuned to the scent trails of the scavenging Argentine ants, no functional association between these two species has ever been reported. Who knows? Maybe it's more than just a coincidence. Even without any permanent association, the Surinam cockroach was found to follow the scent trails of one of its hosts, Camponotus brutus, to a food source. Surely the enterprising American cockroach is capable of doing the same?